My 3 are bigger and are at/went to superselectives. I agree that personality is important as to whether they fit in as well as ability.If a child isn't " a grafter" they are going to find a superselective unpleasant. Pace of teaching and work is fast, so there isn't the repetition etc that you might find elsewhere.
I have no idea "how bright" my kids are /were at entry - we weren't given the grammar test raw scores and SATS were only to level 5- which they all achieved across the board early in year 5. However in the Dds school where they give prizes for over all academic achievement the eldest got the form prize from year 8 to 11 (then there wasn't a similar award in 6th form) so she must have been amonths the 5 brightest in the year I guess. THe youngest is attempting to out do her sis and got the award in year 7 (she's only year 8 now). Boys school doesn't give a comparable award.
The stats I think are that the top 5% academically in the city get into these schools- to me that is still 1 in 20 so not necessarily "genius" material, but it means that only perhaps 4 would get in out of the year group or 90. This was pretty accurate (except DD1s year who were generally able and got 9 kids in).
Enrichment and extra curricular stuff is pretty huge too in my kids lives and the life of the school.
My feelings were that the kids needed exam practice to be able to do the tests early in year 6, but not tutoring that "taught them to pass the test" that would be wrong- I certainly didn't want to have to tutor them long term just for them to keep up.
Haziedoll your neice sounds like she would probably be grammar material but in an area with no grammars the "Comps" are truly comprehensive and cater for all abilities so I wouldn't worry. The problem in areas like ours is that it is like the 1960s- If you are bright but not in grammar school you aren't catered for